#include<bits/stdc++.h>
using namespace std;

void solve(int t){
    int x,y,p,q;
    
    cin>>x>>y>>p>>q;
    
    vector dist(x+1,vector(y+1,array<int,2>{-1,-1}));
    vector ok(x+1,vector(y+1,array<bool,2>{false,false}));
    priority_queue<array<int,4>> pq;
    
    dist[x][y][0]=0;
    pq.push({0,x,y,0});
    
    auto dj = [&](int sheep,int wolves,int lato){
        int ots=x-sheep,otw=y-wolves;
        
        
        if(ok[sheep][wolves][lato])return;
        //cout<<sheep<<" "<<wolves<<" "<<lato<<" "<<dist[sheep][wolves][lato]<<"\n";
        ok[sheep][wolves][lato]=true;
        
        for(int i=0;i<=sheep;i++){
            for(int j=0;j<=wolves && i+j<=p;j++){
                if(sheep-i==0 || sheep-i+q>=wolves-j){
                    //cout<<"provo "<<i<<" "<<j<<" lasciando "<<sheep-i<<" "<<wolves-j<<"\n";
                    if(dist[ots+i][otw+j][lato^1]==-1 || dist[ots+i][otw+j][lato^1]>dist[sheep][wolves][lato]+1){
                        dist[ots+i][otw+j][lato^1]=dist[sheep][wolves][lato]+1;
                        pq.push({-dist[ots+i][otw+j][lato^1],ots+i,otw+j,lato^1});
                    }
                }
            }
        }
    };
    
    while(!pq.empty()){
        auto [a,b,c,d] = pq.top();
        pq.pop();
        
        dj(b,c,d);
    }
    
    int ans=INT_MAX;
    for(int i=0;i<=y;i++){
        if(dist[x][i][1]!=-1){
            ans=min(ans,dist[x][i][1]);
        }
    }
    if(ans==INT_MAX)ans=-1;
    cout<<ans<<"\n";
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int t=1;
    //cin>>t;
    for(int i=1;i<=t;i++)solve(i);
    
    return 0;
}